1. Field of the Invention
The present invention relates to an optical pickup for an optical recording and/or reproducing apparatus which records and/or reproduces information from a disc-type optical recording medium. More particularly, the present invention relates to an optical pickup for recording and/or reproducing information from two or more types of recording media, such as a high density recording medium such as a blu-ray disc and a lower density recording medium such as a digital versatile disc (DVD) and/or a compact disc (CD).
2. Background of the Invention
In an optical recording and/or reproducing apparatus which records information on and/or reproduces information from a disc type recording medium using a light spot that is focused by an objective lens, the recording capacity of the optical disc is determined by the size of the light spot. The size S of a light spot is determined by the wavelength λ of the light and a numerical aperture (NA) of the objective lens, as represented in Equation 1:S∝λ/NA  (Equation 1)
Since the emergence of CDs, where data is recorded and/or reproduced by using a light having a wavelength of 780 nm and an objective lens having a numerical aperture of 0.45 or 0.5, considerable research has been conducted into methods to increase information storage capacity by increasing recording density.
A DVD is a recording medium where data is recorded and/or reproduced by using light having a wavelength of 650 nm and an objective lens having a numerical aperture of 0.6 or 0.65. Blu-ray disc (BD) technology has been recently introduced and is a high density recording medium. Data is recorded and/or reproduced by using light having a relatively short wavelength of 405 nm and an objective lens having a numerical aperture of 0.85.
To help minimize coma aberration (the aberration of a light spot due to tilt of the recording medium) in a blu-ray disc, the light transmission layer of the blu-ray disc is only 0.1 mm thick. In addition, the adoption of an objective lens having a numerical aperture of 0.85 requires that the recording medium have a uniform thickness. In particular, since the amount of a spherical aberration is proportional to the fourth power of the numerical aperture of the objective lens and the deviation in thickness of a recording medium, the recording medium must have a uniform thickness with a deviation range of ±3 mm. It is, however, very difficult to fabricate a recording medium within this deviation range. Therefore, an optical pickup apparatus for a BD has a device for correcting for a spherical aberration that occurs due to deviation in the thickness of a recording medium. Moreover, since the blue laser light used in an optical pickup of a BD device has a comparatively large variation of refraction index in an optical medium, compared with the red laser light used in a CD or a DVD, serious chromatic aberration occurs. Furthermore, when the mode of a laser diode is changed from a reproduction power to a recording power, the wavelength shifts momentarily, for example within several microns, thus also requiring correction of a chromatic aberration.
In addition, existing one-time writable DVD-R and CD-R discs are designed for use with light sources having a wavelength of 650 nm and 780 nm. Since the reflectivity of the disks significantly decreases when different wavelengths are used, it is desirable to provide a light source that generates light with a wavelength of 650 nm and 780 nm.
Accordingly, an optical pickup apparatus for use with both a high density recording medium (such as a BD) and a lower density recording medium (such as a DVD and a CD), should have a device for correcting a spherical aberration (for example, a beam expander), a device for correcting a chromatic aberration (for example, a diffractive optical element), and a light source that emits light beams having two or three different wavelengths.
FIG. 1a shows a conventional optical pickup. The pickup comprises a beam expander 10 to correct a spherical aberration that occurs due to differences in thickness of the disc D. As shown in the drawing, the light paths of light emitted from a first source 11 and a second source 12, respectively, are changed by a first beam splitter 13 and a second beam splitter 14 and are then irradiated to the disc D through the beam expander 10 and the objective lens 15.
Here, the first light source 11 is, for example a laser diode for a BD that emits light having a wavelength of 405 nm. The second light source 12 is, for example, a hologram module for a CD and/or a DVD. Lights emitted from the light sources 11 and 12, respectively, are changed into parallel beams by respective first and second collimating lenses 16 and 17 disposed adjacent to the second beam splitter 14. The parallel beams are then irradiated to the disc D and reflected. The beam expander 10 is driven by an actuator 18 and corrects a spherical aberration that occurs due to variations in the thickness of the disc.
Since the conventional optical pickup which is compatible with different types of media explained above uses the beam expander 10 which occupies a comparatively large installation space and also two collimating lenses 16 and 17, it is very difficult to miniaturize an optical pickup which is compatible with different types of media.
FIG. 1b shows another conventional optical pickup which is compatible with multiple types of media. As shown in the drawing, this optical pickup has three light sources 21, 22 and 23 for emitting lights with different wavelengths, respectively and beam splitters 25, 26, 27 and 28 for changing respective paths of lights emitted from the light sources 21, 22 and 23, toward the objective lens 24. The above light is focused into the disc D by the objective lens 24.
Furthermore, the optical pickup has three collimating lenses 31, 32 and 33 for collimating lights emitted from the light sources 21, 22 and 23, respectively. In addition, in this optical pickup, lights with different wavelengths are changed into parallel beams by the collimating lenses 31, 32 and 33 and are incident on the objective lens 24. The collimating lens 31, which is disposed in a light path of the light source 21 for the BD emitting the light having a short wavelength, is driven in order to correct a spherical aberration. To do so, the collimating lens 31 is moved in the direction of its optical axis. The collimating lens 31 also has an irregular diffractive face to correct a chromatic aberration. Accordingly, though it may be miniaturized more than the optical pickup using the beam expander 10 shown in FIG. 1a, it has three collimating lenses 31, 32 and 33, and thus it is still hard to miniaturize an optical pickup.
Accordingly, there is a need for an improved optical pickup which is compatible with multiple types of media and which can be made smaller.